1. Field of the Invention
The present invention is directed to a developer station for electrophotographic printer and copier devices.
2. Description of the Related Art
In electrophotographic printer and copier devices, a latent charge image is generated by charge differences on a photoconductor such as, for example, a photoconductor of a photoconductive drum. Depending on the method used in the printer or copier, the toner is applied to either the charged or the discharged regions of the photoconductor, this toner being subsequently transferred onto a recording medium such as, for example, a sheet of paper and is then fixed thereon.
In order to be able to apply toner onto a photoconductor, a developer mixture that, for example, is composed of two components, toner and carrier material, is offered in a developer station. In the developer station, the toner is charged by being mixed with the carrier material, which is for example iron oxide, and is applied onto one or more developer drums that usually comprise a rotating magnet rotor in a rotating sleeve. As a result of the magnetic field that is generated by the rotating magnets, what are referred to as toner brushes are formed on the sleeve of the developer drum, these toner brushes being aligned according to the magnetic field.
Due to the rotation of the sleeve of the developer drum, the toner brushes are transported to the photoconductor and are transferred either onto the charged or the discharged regions depending on the method being employed.
U.S. Pat. No. 4,878,089 discloses a developer station in which a two-component developer composed of toner and toner carrier is transferred onto a photoconductor. To this end, a hollow, cylindrical deflection housing having a circular cross section is provided close to the photoconductor, a developer transfer region, at which the developer material is applied onto the deflection housing by a conveyor means, and a toner transfer region, at which the toner is transferred onto the photoconductor, being formed on the surface of this deflection housing. A magnet rotor is eccentrically arranged in the deflection housing such that the spacing of the magnet rotor from the outside surface of the deflection housing decreases toward the toner transfer region proceeding from the developer transfer region. The rotating magnet rotor generates a migrating magnetic field at the surface of the deflection housing, the developer on the surface of the developer transfer region being conveyed to the toner transfer region by this migrating magnetic field. The uniform minimization in the spacing of the magnet rotor thereby effects a continuous increase in the magnetic forces toward the toner transfer region proceeding from the developer transfer region, as a result whereof the developer forms uniformly thick toner brushes in the toner transfer region.
In the known developer station, it is especially disadvantageous that a toner transfer from the deflection housing onto the photoconductor can be non-uniform since the transfer of the toner from the deflection housing to the photoconductor ensues only in a narrow, line-like transfer region. Further, the space required for the above-described developer station is relatively great.
For transferring developer (toner) onto a photoconductor, U.S. Pat. Nos. 5,080,038 and 5,181,075 provide a stationary, non-magnetic housing in which a magnet rotor likewise rotates. As a result of the rotating magnet rotor, an alternating magnetic field with which, above all else, toner is conveyed along the stationary, non-magnetic housing is generated. The part of the stationary housing that lies opposite the photoconductor such as, for example, a photoconductive drum which is fashioned complementary thereto.
Despite the described fashioning of the stationary housing, the embodiments disclosed in the two United States patents have the disadvantage that the magnetic field is weaker at the two edges of the toner transfer region from the stationary housing to the photoconductive drum since the distance between rotating magnet rotor and stationary housing is larger at the region edges.
In order to avoid the above-described disadvantage, magnetizable spaces (see U.S. Pat. No. 5,181,075) and weak permanent magnets additionally referred to as transport-supporting magnets (see U.S. Pat. No. 5,080,038) are provided in the edge regions (referred to as shoulders) of the toner transfer region in which the magnetic field is weaker. The field lines are intensified by these two magnets but this also leads to increased eddy current losses. Higher eddy currents are likewise to be expected since the magnet rotor is not eccentrically arranged, this resulting a heating of the housing and leading to an adhesion of toner to the housing. Further, the magnetic field is non-uniformly formed on the surface of the housing.